Since I'm used to CSS, where I get 70-100 fps, and fps is way more important to me than graphics, I'm playing at 800x600 on low and probably between 30-50 fps for the most part. The highest my fps got to was 70, the lowest was 15 during a huge firefight, which call of duty 2 also got me down to about that low during the busiest parts of the game.
I did play with 1024x768 resolution but it lowered my fps to being consisitently below 30 fps, which I hate, with my outdated rig I'm just into modern games for the gameplay and not graphics at this point.